linux-ext4 - Re: Unexpected writes seen on a Read-only, DAX enabled EXT4 fs

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Message-Id: <9F35007B-F0D9-4FB9-A52D-604E410AB389@dilger.ca>
Date:   Mon, 11 Sep 2017 14:31:41 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Soccer Liu <soccer_liu@...oo.com>, linux-ext4@...r.kernel.org,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: Unexpected writes seen on a Read-only, DAX enabled EXT4 fs

On Sep 11, 2017, at 12:21 PM, Ross Zwisler <ross.zwisler@...ux.intel.com> wrote:
> 
> On Sat, Sep 09, 2017 at 05:10:26PM +0000, Soccer Liu wrote:
>> Hi:
>>   I was surprised to see a few write operations called into
>> ext4_dax_huge_fault with FAULT_FLAG_WRITE set on vmf->flags
>> Are those writes expected at all?
>> static int ext4_dax_huge_fault(struct vm_fault *vmf,
>> enum page_entry_size pe_size)
>> {
>> ...
>> bool write = vmf->flags & FAULT_FLAG_WRITE;
>> Based on the following Mount documentation below, I did mount this ext fs
>> with ro and noloadAre there any other options that I could use to totally
>> eliminate those unexpected writes?
>> 
>>   - -r, --read-only
>>      - Mount the filesystem read-only. A synonym is -o ro.Note that,
>>      depending on the filesystem type, state and kernel behavior, the
>>      system may still write to the device. For example, Ext3 or ext4 will
>>      replay itsjournal if the filesystem is dirty. To prevent this kind of
>>      write access, you may want to mount ext3 or ext4 filesystem with
>>      "ro,noload" mount options or setthe block device to read-only mode,
>>      see command blockdev(8).  -
>> 
>> Related DMESG output...
>> 
>> [    0.891653] EXT4-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
>> [    0.892736] EXT4-fs (pmem0): mounted filesystem without journal. Opts: noload,dax
>> [    0.898293] EXT4-fs (sda): mounted filesystem without journal. Opts: (null)
>> [    0.902574] random: fast init done
>> [    0.926157] IPVS: ftp: loaded support on port[0] = 21
>> // My private debug messages
>> [    0.997224] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2 <--read-only device
>> [    0.998052] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
>> [    0.998787] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
>> [    0.999720] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [    1.004626] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [    1.005359] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [    1.006128] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [    1.006824] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [    1.007499] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [    1.008149] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [    7.320626] random: crng init done
>> [   18.780447] EXT4-fs: 53 callbacks suppressed
>> [   18.781900] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [   18.783392] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [   18.790661] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [   18.792042] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [   18.793321] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [   18.795005] EXT4-fs (pmem0): write is 0 handle = 0x0
>> [   18.795204] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
>> [   18.795209] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
>> [   18.795213] EXT4-fs (pmem0): write is 1 handle = 0xffffffffffffffe2
>> [   18.795257] EXT4-fs (pmem0): write is 0 handle = 0x0
> 
> Adding the ext4 list.
> 
> I'm pretty sure that this is ext4/mm behavior that is independent of DAX.  DAX
> doesn't really have any impact on whether or not write page faults happen on a
> RO ext4 filesystem, though this fix from Randy Dodgen:
> 
> https://www.spinics.net/lists/linux-ext4/msg58075.html
> 
> shows that FAULT_FLAG_WRITE can be set on DAX read-only ext4 mounts when
> executing binaries, though in this case the write is going to a COW page.

Also, ext4 journal recovery will be done on the filesystem regardless of
whether it is mounted read-only or not.

Cheers, Andreas






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